[gpaw-users] 2D Metal Sheets

[Tomlinson, Warren (CDR)]

All-
	I have run into a curious result with my 2D magnetic sheet calculations and I was thinking you might be able to help. I have done calculations on the following 2D sheets:
NiCl2
NiBr2
CoCl2
FeCl2
MnCl2
MnBr2

Step one was to find the correct geometry and cell size. I did this with the following code:

for a in [6.87, 6.88, 6.89, 6.90, 6.91]:

   sheet = mx2(formula='FeCl2', kind='1T', a=a/2, thickness=2.5, size=(2,2,1), vacuum=17)
   sheet.set_pbc([1,1,1])
   name = '%.2f' % a

   calc = GPAW(mode=PW(600),
           kpts=(10,10,1),
           xc='PBE',
           parallel={'kpt': 30, 'band': 1},
           basis = 'dzp',
           convergence={'energy': 0.0001},
           occupations=FermiDirac(0.1),
           setups={'Fe': ':d,3.0'},
           txt=name + '.txt',
           maxiter=1000
           )

   sheet.set_calculator(calc)
   opt = BFGS(sheet)
   opt.run(fmax=0.01)
   E = sheet.get_potential_energy()
   parprint('**********')
   parprint(name + ':\t %f' % E)
   parprint('**********')
   write(name + '.traj', sheet)

—————

Next I did spin polarized calculations to determined what (if any) magnetic state was most preferred. Here is the code for that:

a = read('FeCl2.traj')
mm = [[1.5, 0.1, 0.1, 1.5, 0.1, 0.1, 1.5, 0.1, 0.1, 1.5, 0.1, 0.1],
     [-1.5, -0.1, -0.1, 1.5, 0.1, 0.1, 1.5, 0.1, 0.1, -1.5, -0.1, -0.1],
     [0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0.]]
type = ['FM', 'AFM', 'None']
name = 'FeCl2'
E = 0

for t in type:
   if t == 'FM': a.set_initial_magnetic_moments(mm[0])
   if t == 'AFM': a.set_initial_magnetic_moments(mm[1])
   if t == 'None': a.set_initial_magnetic_moments(mm[2])

   calc = GPAW(mode=PW(600),
           kpts=(10,10,1),
           xc='PBE',
           spinpol=True,
           parallel={'kpt': 30, 'band': 1},
           basis = 'dzp',
           convergence={'energy': 0.0001},
           occupations=FermiDirac(0.1),
           setups={'Fe': ':d,3.0'},
           txt=t + '_' + t + '.out',
           maxiter=1000
           )

   a.calc = calc
   energy = a.get_potential_energy()
   parprint('$$$$$$')
   parprint('E for ' + t + ' mode: %f' % energy)
   parprint('$$$$$$')

My results show that FM is preferred for all but FeCl2, MnCl2 and MnBr2. This matches the experimental results from another group I am working with, except for FeCl2. While they show the Mn structures preferring an AFM state, they show FeCl2 preferring an FM state. The results I got for FM vs AFM for FeCl2 are not even close. AFM is about 1.6 eV lower in energy than FM. This is a considerably larger difference then what I found for the other structures. The other structures had FM-AFM energy differences of around 50-100 meV, though the CoCl2 difference was relatively high at ~700 meV. Can you think of any reason why my results for FeCl2 are so far off? Should I be treating Fe differently for some reason? Am I missing an important element of my spinpol calculations that I’m just not seeing?
Thanks,
Warren


CDR Warren Tomlinson, Ph.D.
Department of Physics
Naval Postgraduate School
Sp. 118
(831) 656-2877
wwtomlin at nps.edu <mailto:wwtomlin at nps.edu>


CDR Warren Tomlinson, Ph.D.
Department of Physics
Naval Postgraduate School
Sp. 118
(831) 656-2877
wwtomlin at nps.edu




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